Two numbers you cannot guess and cannot get right by trial and error on site. Give the spring angle of the moulding and the actual corner angle, and this gives you the miter and bevel to dial into the saw.
Notes
Why 45 degrees does not work
Crown sits on the wall at a spring angle, so the cut face is not square to the saw table. The miter and bevel both shift, and for standard 38 degree crown on a square corner they land at 31.6 and 33.9, not 45 and 0.
Measure the corner, do not assume
Walls are rarely 90 degrees. A corner that is 2 degrees out leaves a visible gap at the top of the moulding. A digital angle finder pays for itself on the first room.
The formulas
miter = atan(sin(spring) x tan(corner/2)), bevel = asin(cos(spring) x cos(corner/2)). Both are shown in the results so you can check them.
What this does not do
It does not know which way up your moulding goes on the saw. Cut a test piece in scrap first, every time. Nobody gets the orientation right from memory.